A RECONNAISSANCE OF MICROPLASTIC DISTRIBUTION IN A SMALL MICHIGAN WATERSHED

While microplastics in the world’s oceans have received significant attention, there has been less study of plastics in other environments such as streams and rivers. We sampled sediments from fluvial and lacustrine environments in the Lake Macatawa watershed to reconnoiter the distribution and quantity of microplastics and identify potential sites of microplastic deposition. Sampling sites included three streams, with upstream woodland/agricultural, midstream residential, and downstream urban localities; a Lake Michigan beach at Holland State Park; the mouth of a wastewater treatment plant outfall; and Typha wetlands surrounding two storm sewer outfalls. Samples were collected via 2-inch sediment cores or a Petite Ponar sampler depending on site conditions.

An elutriation tower that was used to process sand samples spiked with known quantities of high density microplastics (PVC and PETE, >1.3 g/cm³) yielded 100% recovery. This tower was used to sort plastic from sand in the beach samples. Stream and sewer outfall samples, however, were too organic- and silt-rich for elutriation to be effective and were processed by wet peroxide oxidation and density separation using ZnCl or Na polytungstate.

Macroplastics were observed in the sediment at most sampling sites, but relatively low microplastic counts were found in stream samples (maximum of 2), with larger counts present in beach samples (maximum of 5) and storm sewer outfall samples (maximum of 7), indicating the possibility that plastics are washing through the system relatively quickly and accumulating in sediment depositional areas. The sediment cores sample a relatively small surface area (18.28 cm²) and volume (maximum of 570 ml) of sediment, so quantifying reliable estimates of microplastic concentrations will require more repeated samples from each site than this reconnaissance allowed, but the volume of subsamples processed from each sample typically did not exceed 100 ml of sediment, which makes the presence of plastics, even on such a small scale, concerning. Further investigation is required to determine sources and final destinations for these plastics, but results indicate that upstream sources do exist.